[Four Corners] Scriptura?

There is a classical dispute in the law of contracts, the underlying problem of which also bears on the doctrine of sola Scriptura.  

Suppose you enter into a contract to purchase a home from a seller, and at some point the other party refuses to sell, claiming that a term of the contract allows them out of the deal.  If you disagree with their interpretation of the disputed clause, and take it to a judge, what should he consider in resolving the matter?  Should he only consider the "four corners" of the contract that you and the seller signed (i.e., nothing beyond the written page itself), or should he also consider extrinsic ("parol") evidence, such as testimony that the seller assured you orally that the clause meant the opposite of what he now claims?   

The underlying issue, then, is whether courts can consider something more than the contract, when the parties are bound only by the contract document they signed.  The traditional position has been that only the written contract could be considered by the courts, not testimony about oral promises made outside of the writing.  If we wax theologic, this is sola pactum, if you would.  But a rift started to emerge in the courts, prompted, as is often the case, by bad cases and clear scoundrels benefiting from a 'bright-line' rule.  

A fairly philosophical view appeared: a contract is never in fact interpreted by its four corners alone because every judge's mind comes pre-loaded with normative or interpretive biases.  Anyone reading it would filter the words on the page through their own preexisting understanding of language, or of the matters being agreed upon in contract.  Specialty terms from a particular field related to the contract (e.g., construction terms) may have a different meaning to the contracting parties than they would to a lay judge.  Language is never a perfect medium for underlying thought, it seems.

Analogously, if our authority for faith and morals is the Bible alone, may we look only to the four corners of Scripture, or do we admit extrinsic sources to our interpretation as well? Some will insist upon a negative answer: "no book but the Bible, no creed but Christ."  

The Reformer opposed to 'biblicism' will be quick to note that his authority is sola Scriptura interpreted with the church (see here).  That is, some measure of deference to others' interpretations or to a traditional vein of interpretation is due.  This view, which I admire for its humble respect for tradition, is the analog to the liberal trend in contract interpretations admitting extrinsics.  But in choosing our extrinsics, in selecting whose or which traditional vein's interpretation receives our deference, we, like judges and anyone else handling text, do not start with an interpretive tabula rasa.  We add our own extrinsic.  And like the specialty terms in contracts worsening the problem of four corners alone for judges, specialty terms, period-specific terms and the like, in the Bible worsen the problem when attempting to interpret Scripture without the influence of pre-loaded biases.

How is the term sola proper when it is not [Four Corners] Scriptura, but Scriptura + Interpretive Extrinsics?